# Metallization without Charge Transfer in CuReO4 Perrhenate under Pressure

## TL;DR

This study shows how CuReO4 becomes metallic under pressure without charge transfer between its elements.

## Contribution

The paper reveals metallization in CuReO4 under pressure occurs via band widening, not charge transfer.

## Key findings

- CuReO4 undergoes two first-order phase transitions at 1.5 GPa and 7 GPa.
- The high-pressure phase of CuReO4 is metallic, as predicted by density-functional calculations.
- Metallization occurs due to increased bandwidth of Cu 3d and Re 5d bands without significant charge transfer.

## Abstract

Using high-pressure
synchrotron X-ray diffraction combined with
Raman spectroscopy and density-functional calculations, we determined
the sequence of the pressure-induced transformations in CuReO4. At 1.5 GPa, the lattice symmetry changes from I41cd to I41/a with the transformation of isolated ReO4-tetrahedra into infinite chains of ReO6-octahedra. The
second, isosymmetric transition at 7 GPa leads to the formation of
a NbO2-type structure with the octahedral oxygen coordination
for both Cu1+ and Re7+ cations. Both transitions
are of the first order and accompanied by discontinuities in the unit-cell
volume of 7 and 14%, respectively. Density-functional calculations
predict the metallic state of the high-pressure NbO2-type
phase of CuReO4, and this prediction is in-line with the
disappearance of the Raman signal above 7 GPa and visual observations
(darkness/reflection of the sample). This metallization is caused
by the increased bandwidth of both Cu 3d and Re 5d bands without any
significant charge transfer between Cu and Re. At ambient pressure,
the crystal structure of CuReO4 is retained between 4 and
700 K (melting point), showing a negative thermal expansion along
the c-axis and a positive expansion along the a-axis within the entire temperature range.

Using high-pressure synchrotron X-ray diffraction combined
with Raman spectroscopy and density-functional calculations, we determined
the sequence of the pressure-induced transformations in CuReO4.